Description of experiment
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Needed compounds:
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hydrochloric acid : HCl
cupric oxide : CuO
citric acid : COOH CH2 COH(COOH) CH2 COOH
Class:
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elem=Cu
acid/base
coordination
Summary:
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Copper(II) oxide slowly dissolves in a highly concentrated solution of citric
acid and after a while a brightly colored crystalline solid is formed, which
most likely is copper(II)citrate.
Description:
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Take a big spatula full of citric acid and add a little amount of finely
powdered black copper(II) oxide. A large excess of citric acid is used. Add an
equal amount of water and shake: Only part of the citric acid dissolves and the
copper(II) oxide does not dissolve.
Heat the solution, such that the liquid starts boiling gently and the solution
is swirled well during the heating: All of the citric acid dissolves, making
this a very strongly concentrated solution. The copper(II) oxide only dissolves
very slowly and even after several minutes of heating, still some copper(II)
oxide remains left undissolved. The solution becomes very dark green with a
bluish hue.
Keep on heating for a longer time: More of the copper(II) oxide dissolves but
at the same time a crystalline green solid is deposited and the liquid becomes
lighter again. After a while, the liquid has a nice blue color with a faint
green hue, there is a lot of a fine crystalline fairly dark green powder, which
quickly settles at the bottom and there also is some black copper(II) oxide
left, which forms a cake which sticks at the glass.
Let the liquid cool down a little and then add a few ml of concentrated
hydrochloric acid: The green solid almost immediately dissolves and the
solution becomes yellow/green. The black cake of copper(II) oxide also
dissolves, but this takes a little longer.
Pour the liquid in a large excess of cold water: A pale blue clear liquid is
obtained.